The present invention concerns a method of determining variations in the morphology of a borehole.
Electrical imaging of the borehole wall is increasingly being used to determine the shape and the dimensions of said borehole. Electrical images of the borehole wall are obtained by means of special-purpose tools known as dipmeters including the FORMATION MICRO SCANNER (FMS) developed by SCHLUMBERGER.
A dipmeter such as the FMS has at its lower end means for emitting a focused electric current and pads which bear against the borehole wall. The electrical resistivities of said wall are measured using electrodes on each of said pads. The number of electrodes on each pad can vary from one tool to another, in order to obtain better coverage of the borehole wall.
The FMS or equivalent measurement tool is designed to operate in conductive water-based mud, the scanning depth varying from 2.5 cm (1 inch) to 15 cm (6 inches).
The tool has at its upper end at least one three-axis accelerometer and three magnetometers for measuring the speed, the position and the orientation of the tool in each measurement period.
The logged measurements obtained using the tool can be processed, for example to correct the speed and in particular to correct irregular movement of the tool due to the tool jamming in the borehole, and to correct the current since the current emitted varies to maintain the optimal resolution in the event of high contrast in the resistivity.
Further processing can also be applied, such as horizontal normalization of the measurements and representation of the resistivity images relative to a given azimuth direction, usually North.
The measurement tool described in outline above is used, among other things, to determine the direction of maximum stress that can deform the wall of the borehole. The generic term for this is "ovalization".
Due to in situ stresses of tectonic origin, the wall of a borehole tends to scale in a preferred direction, creating eccentric voids of greater or lesser depth with their major axis perpendicular to the direction of the maximum horizontal stress. This phenomenon is known as "ovalization by scaling".
If the direction of the maximum horizontal stress is known, the directions in which cracked reservoirs drain are known more accurately, it is possible to predict the directions in which hydraulic fractures develop, and some wall strength problems can be understood and solved.
Accordingly, attempts have been made to develop ovalization as a means of detecting, orienting and quantifying deformation of the wall of a borehole, and above all to discriminate between the various types of ovalization to determine that which can indicate the orientation of the maximum horizontal stress, since there are several types of ovalization that are related to other types of deformation, of greater or lesser apparent similarity, but of different origins. These include wear ovalization due to rubbing of the string of drill pipes against the wall of the borehole, ovalization due to the presence of structural discontinuities such as stratification, fracturing, etc.
Unfortunately, existing techniques are not able to discriminate or to distinguish quickly and reliably the type of ovalization which can indicate the orientation of the maximum horizontal stress.